Microwave transmission-line t-filters



Nov. 12, 1968 P. E. SCHMID ETAL 3,411,114

MICROWAVE TRANSMISSION-LINE T-FILTERS Filed April 50, 1965 5 Sheets-Sheet 1 3 INVENTORS.

PIERRE E. SCHMID HEINZ M. SCHLICKE HORACE S. DUDLEY RICHARD F. NEUENS Nov. 12, 1968 P. E. SCHMID ET AL 3,411,114

MICROWAVE TRANSMISSION-LINE T-FILTERS Filed April 30, 1965 5 Sheets-Sheet ATTENUATION IN db.

ATTENUATION IN db.

INVENTORS. PIERRE E. SCHMID HEINZ M. SCHLICKE HORACE S. DUDLEY RICHARD E NEUENS Nov. 12, 1968 P. E. SCHMID ET AL MICROWAVE TRANSMISSION-LINE T-FILTERS 5 Sheets-Sheet 5 Filed April 30, 1965 ANTENNA DIPLEXER FILTER FREQUENCY TRANS. 2

TRANS. I

INVENTORS. PIERRE E. SCHMID HEINZ M. SCHLICKE HORACE S. DUDLEY RICHARD F. NEUENS BY $.fl/

Nov. 12, 1968 P. E. SCHMID ET AL 3,411,114

MICROWAVE TRANSMISSION-LINE T-FILTERS Filed April 30, 1965 5 Sheets-Sheet 4 I j A02 1 2- m -a: 3 2 f2 .6'/ 2 a 2 41 52 lNPUTl OUTPUT INPUT 2 INVENTORS. PIERRE E'. SCHMID HEINZ Mv SCHLICKE HORACE S. DUDLEY RICHARD F. NEUENS BY M/ Nov. 12, 1968 P. E. SCHMID ET AL 3,411,114

MICROWAVE TRANSMISSION-LINE T-FILTERS Filed April 50, 1965 5 Sheets-Sheet 5 Q i z 30" m g #5 s a O z 5 g (I IT I I I g I z W UJ ,t N

lO-- g CHANNEL 3 m CHANNEL m (n a a 1.0 -o.a db.

FO! "F02 FREQUENCY INVENTORS. PIERRE E. SCHMID HEINZ 'M. SCHLICKE HORACE S. DUDLEY RICHARD F. NEUENS United States Patent 3,411,114 MICROWAVE TRANSMISSION-LINE T-FILTERS Pierre E. Schmid and Heinz M. Schlicke, Fox Point, Horace S. Dudley, Milwaukee, and Richard F. Neuens, Waukesha, Wis., assignors to Allen-Bradley Company, Milwaukee, Wis., a corporation of Wisconsin Filed Apr. 30,1965, Ser. No. 452,049 20 Claims. (Cl. 33373) ABSTRACT OF THE DISCLOSURE This invention relates to a microwave filter and there is shown and described a microwave filter comprising an electrically T-shaped configuration made up of three microwave transmission lines. Two transmission lines branch out from a junction common with one end of the third transmission line, and the other end of the third transmission line forms a terminal for connection with a signal propagating line that comprises an input line from a signal source and an ouptut line extending to an electrical load. The filter of the invention is thus shunt connected to the line for which it is to perform a filtering function. The invention further teaches that a plurality of the above-described T-filters may be connected in parallel or cascade to form a multi-channel filter network.

This invention relates to a microwave filter and more specifically to a microwave filter comprising a substantially T-shaped configuration of three microwave transmission lines. Two transmission lines branch out from a shunt junction common with one end of the third transmission line and the other end of the third transmission line leads to another shunt junction common with the input line from the signal source and the output line to the electrical load of the filter. The invention further teaches that a plurality of the above-described T-filters may be connected in parallel or cascade to form a multi-channel filter network.

With increasing use of communication installations employing multiple channels operating at microwave carrier frequencies, the need for filtering and separating such carrier frequencies in apparatus handling large amounts of power has presented an acute problem. A commonsituation is a communication installation having a plurality of microwave transmitters feeding into a common antenna. Each transmitter operates at a distinct carrier frequency and comprises an individual channel. To avoid interference between the various channels a filter is required at the output of each transmitter to pass the particular channel frequency to the antenna and to block the frequencies of the neighbor channels from feeding back into the transmitter. Filtering may also be required at the receiver installation. A plurality of receivers may be located at a common installation, each tuned to a different channel frequency. Clear reception requires that all channels other than that to which each receiver is tuned be filtered and blocked from passage to that particular receiver.

The design of such filtering networks demands consideration of the flolowing requirements: First, a steep rise in attenuation near the channel pass band, commonly referred to as high skirt selectivity, is necessary because the carrier frequencies are usually spaced very close-two percent of channel center frequency is typical channel 3,411,114 Patented Nov. 12, 1968 separation. Second, the filter must be capable of handling large amounts of power of which only a minimum amount is allowed tobe dissipated in the filter. In other words, low insertion loss in the pass band is required. Third, many applications such as in space telemetry, require a filter with small physical dimensions and the lowest possible weight. Furthermore, depending on the particular application, the filter may be called upon to operate under adverse environmental conditions, such as changing temperature, pressure, acceleration, shock and vibrations.

In the microwave filters heretofore available all of these requirements cannot be realized at the same time. The present invention provides a versatile filter that meets these requirements. The filter comprises three transmission lines, connected in a shunt-T arrangement with the trunk member of the T serving as an admittance transforming line and the two arm members of the T branching out from the upper end point of the trunk member. The arm members may terminate in an open or short circuit relationship.

The T-filter is connected in shunt with the signal source and the load at the lower end point of the trunk member. Therefore, if the T-filter is regarded as a two-terminal pair network operating in a system with matched source and load impedances, the equivalent circuit of the T-filter arrangement comprises a transmission-line T with the lower trunk end connected in shunt across the common matched input-output line.

The frequency response of the transmission-line T filter is determined by the electrical length, characteristic impedance and attenuation constant of the three lines. For purposes of explanation the center frequency for a band pass filter is that frequency at which the attenuation in the pass bend is minimum and for a band rejection filter that frequency at which the attenuation is maximum in the rejection band.

From the design point of view, the following qualitative statements can be given about the present structure for a band pass T-filter: First, the pass band center frequency is primarily dependent on the electrical length and characteristic impedance of the two arm members. More specifically, if the characteristic impedance of each of the two arm members is equal, the center frequency is primarily determined by the mean electrical length of the two arm members. Second, the insertion loss in the pass band is dependent primarily on (a) the electrical length ratio of the two arm members and (b) the attenuation constant of the two arm members. Third, although the absolute position of the rejection band depends on the electrical length and characteristic impedance of all three lines, the relative rejection band location (above or below the pass band) is determined by the electrical length and characteristic impedance of the third admittance transforming line.

The present T-filter is not limited to band-pass type filters. It is possible to transform the band-pass T-filter into a band-rejection T-filter by choosing the electrical length of the third line as an odd multiple of quarter wavelengths, with respect to the pass band center frequency of the band-pass T-filter. As it is well known from transmission line theory a quarter-wave line transforms an attenuation pole into a zero impedance and vice versa, such that the rejection band of the band-rejection T-filter centers about the initial center frequency of the band-pass T-filter.

As a band-pass filter, the slope of the attenuation curve can be made very steep near the pass band while maintaining low insertion loss. This permits wide applications of the T-filter structure in microwave networks demanding high rejection in the vicinity of the pass band, whether it be above or below the pass band, with very low insertion loss in the pass band. In many multi-channel microwave systems, high attenuation is required-over only a limited bandwidth. For example, in a two-channel, microwave, transmitter installation, the rejection bands of the filter associated with each transmitter need be just broad enough to reject the (relatively narrow) frequency spectrum of the neighbor transmitters. Therefore, the conventional band-pass filter (with two broad rejection bands below and above the pass band) can be replaced by a considerably smaller band-pass T-filter of the present invention designed to have a single rejection pole coinciding with the frequency of the neighbor transmitter. Conventional band-pass filters have the disadvantage that a low pass-hand insertion loss together with high skirt selectivity is realizable only by cascading two or more band-pass filters comprising extremely high-Q resonators which do not meet the requirement of small physical dimensions.

The present T-filter is readily adaptable to a multichannel filtering network wherein a plurality of filters are incorporated in a single structure. The channels are usually interconnected by means of quarter-wave transmission-lines. To acquire a higher skirt selectivity and broader pass bands and rejection bands, the T-filters in each channel may be cascaded using quarter-wave transmission-lines.

Accordingly, an object of the present invention is to provide a versatile microwave transmission-line T-filter which by proper selection of the electrical length of the transmission lines functions as a band-pass or bandrejection filter.

Another object of the present invention is to provide a microwave transmission-line T-filter of which the frequency response is variable according to the length and characteristic impedances of the three transmission lines.

Another object of the present invention is to provide a microwave transmission-line T-filter of which the bandwidth within the pass band and rejection band is variable according to the length and characteristic impedances of the transmission lines.

Another object of the present invention is to provide a microwave transmission-line T-filter in which a rejection pole may be located very close to a pass band while maintaining small insertion loss in the pass band.

Another object is to provide a microwave transmissionline T-filter of which the rejection-pole may be located below or above the pass band.

Another object of the present invention is to provide a filter network incorporating a plurality of cascadeconnected microwave transmission-line T-filters which provides added attenuation in the rejection band and increased bandwidth with respect to a single transmissionline T-filter.

Another object is to provide a microwave transmissionline T-filter which may be designed to small physical dimensions and low weight.

Another object is to provide a microwave transmissionline T-filter which is suitable for the incorporation in a multi-channel filter network requiring a single structure of small physical dimensions and low weight.

Another object is to provide a transmission-line microwave filter which may be designed to operate under adverse environmental conditions.

The foregoing principles and objects of the present invention will appear in the description to follow. In the description reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration specific embodiments in which this invention may be practiced. These embodiments will be described in sufficient detail to enable those skilled in the art to practice this invention, but it is to be understood that other embodiments of the invention may be used and that changes may be made in the body of the invention without deviation from the scope of the invention. Consequently, the following detailed description is not to be taken in a limiting sense; instead, the scope of the present invention is best defined by the appended claims.

In the drawings:

FIG. 1 is a perspective view of a single T-filter in which the two arm members terminate in an open circuit relationship and are positioned in a common coaxial cavity, while the admittance transforming line comprises a miniature coaxial cable. Parts of the structure are broken away to illustrate the internal features of the filter.

FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 1.

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 1.

FIG. 4 is a plurality of attenuation curves illustrating the response of the T-filter of FIG. 1. The curves illustrate the response when the length of the admittance transforming line is equal to or greater than a half wavelength with respect to the center frequency.

FIG. 5 is similar to FIG. 4 and illustrates the attenuation response when the length of the admittance transforming line is equal to and less than a half wavelength with respect to the center frequency.

FIG. 6 is similar to FIG. 4 and illustrates the attenuation response when the length of the admittance transforming line is very short, deviating between zero and a quarter wavelength with respect to the center frequency. The diagram further illustrates that when the electrical length of the admittance transforming line is a quarter wavelength, the filter functions as a bandrejection filter.

FIG. 7 is a plurality of attenuation curves similar to those of FIG. 5 except that the length of the two openended lines differs from that of FIG. 5.

FIG. 8 is a diagram of a communication installation having two transmitters feeding into a common antenna and illustrating an application of the T-filters in a diplexer filter.

FIG. 9 is the typical attenuation diagram of a diplexer arrangement shown in FIG. 8. The diagram includes curves of two single filters of FIG. 1 connected in parallel. The pass band of each channel coincides with the rejection band of the other channel.

FIG. 10 is a perspective, partially broken away view of a diplexer filter incorporating two parallel-connected T-filters of FIG. 1. Each filter is designed to accommodate one channel of the installation shown in FIG. 8.

FIG. 11 is an equivalent circuit diagram of the diplexer shown in FIG. 10. Each length represents the electrical length of a distinct microwave transmission line and each junction represents a parallel (shunt) connection of three transmission lines.

FIG. 12 is a perspective view of a diplexer incorporating two identical T-filters connected in cascade for each of the two channels.

FIG. 13 is an equivalent circuit diagram of the diplexer of FIG. 12.

FIG. 14 is a typical attenuation diagram of the diplexer of FIG. 12.

FIG. 15 is a perspective view of a single stripling T-filter according to the present invention.

FIGS. 1, 2 and 3 illustrate in perspective and crosssection a single bandpass T-filter, referred to by the general reference character 1. In general, and for purposes of explanation, the single T-filter is designed with respect to a center frequency f The filter 1 may be viewed as a two-terminalpair network. The definitions for two-terminal-pair microwave-networks is given by R. M. Fano and A. L. Lawson in Microwave Transmission Circuits,

1948, McGraw-Hill (Chapters 9 and Each input terminal is individually connected to an output terminal and the input-output junction serves as a common shunt connection with the admittance transforming line.

More specifically, input signals to the filter 1 are received at a coaxial input connector 2 having a metallic casing 3 and a center lead 4. The lead 4 is insulated from the casing 3 by an insulating body 5. At the opposite end of the lead 4 is a connecting terminal 7 (best illustrated in FIG. 2). The connecting terminal 7 is the common input-output terminal joining the input lead 4 and an output lead 8. The :output lead 8 is shown in- FIG. 1 as being the center conductor of a short coaxial cable referred to by general reference character 9. The coaxial cable 9 carries an outer conductor 10' attached at one end to the metallic casing 3 and at the other end to a coaxial output connector 11. The output connector 11 is comprised of a metallic casing 12 electrically common to the outer conductor 10. Within the coaxial cable 9 and the coaxial connector 11 is an insulating medium 13 electrically in sulating the center conductor 8 from the conductor 10. The casing 3 is attached to a metallic block member 15 by means of a plurality of screws 16 such that the block 15 is a common electrical point for the filter 1. In applications requiring minimum weight the block 15 may be composed of an aluminum alloy or other lightweight conducting material.

Extending longitudinally through the block 15 is a cavity 17, which cavity preferably has a smooth internal surface for minimal conductor losses. Within the cavity 17 and electrically insulated therefrom is a center conductor referred to by the general reference character 18. The conductor 18 has a pair of arms 19 and 20 extending concentrically within the cavity 17. The relationship of the internal surface of the cavity 17 and the center conductor arms 19 and 20 is such that electrically they appear as separate coaxial TEM transmission lines with air dielectric. The length of the arms 19 and 20 will be designated l and respectively and as will hereinafter be discussed these lengths are significant factors in determining the frequency response of the T-filter.

For variable temperature environments, the center-conductor 18 should be comprised of a material having a low coefiicient of thermal expansion. For example, I-nvar (a nickel-iron alloy) has proven quite satisfactory, and for the purpose of high Q, it is common practice to silver plate the inner surface of the coaxial cavity 17 and the outer surface of the center conductor 18.

Intermediate to the arms 19 and 20 is an electrical conducting member 21. One end of the member 21 is mechanically and electrically attached to the arms 19 and 20 and the opposite end is connected to a center conductor 23 of a coaxial cable referred to by the general reference character 24. The coaxial cable 24 has an outer conductor 25 electrically insulated from the center conductor 23 by means of an insulating medium 26. The combination of the coaxial cable 24 and the member 21 serve as the third line of the transmission-line T-filter. This third line will be generally referred to as the admittance transforming line having an electrical length 1 The length 1 is significant in determining the position of the rejection poles relative to the pass band. The member 21 is mechanically supported and electrically insulated from the block 15 by means of a circular insulating center post 27. Extending coaxial-1y through a substantial portion of the center post 27 is an aperture 2 8 in which the center member 21 is supported. Transverse to the axis of the member 27 and intersecting the aperture 28 is a second aperture 29 which receives the intermediate point of the arms 19 and 20. Thus, the common shunt junction between the member 21, the arm 19 and the arm 20 of the center conductor 18 coincides with the intersection of the apertures 28 and 29. Therefore, for minimum junction capacitance, it is desirable that the dielectric constant of the center-post material be as low as possible.

About the open end of each of the arms 19 and 20 is a tuning cup 31 preferably comprising a lOlW dielectric constant material. Each cup 31 extends between its associated arm 19 or 20 and the internal periphery of the cavity 17. The cups 31 in combination with their associated arms and the cavity 17 act as small tuning capacitors which are movable coaxially along the arm members thereby slightly altering the efiective electrical length of the associated arm member. About the end periphery of each tuning cup 31 is a slot 32 adapted to engage a tuning tool. By varying the position of the tuning cups 31, the preferred frequency response can be precisely set.

It may be noted that the cavity 17 within the housing 15 extends beyond the end of the arms 19 and 20. The cavity extensions in effect serve as circular waveguides operating in their cutoff region and prevent radiation losses from the two open-ended arm members 19 and 20. Generally an extension of the same order of magnitude as the cavity diameter is satisfactory for this purpose.

Many applications of microwave filters expose the filter to contaminated atmospheres, e.g. dust and moisture. The structure of FIG. 1 is designated to facilitate such applications. About each terminal end is an end plug 35 which engages the internal end surfaces of the cavity 17. The end plugs 35 carry an -O-ri-ng 36 hermetically sealing the cavity 17 from the atmosphere. Also, the center post 27 is supported by a discoidal cap 37 which engages the outer surface area of the center post. The discoidal cap 37 has an opening 38 extending transverse to its axis and engaging the outer conductor 25 of the coaxial cable 2 4. Completing the T-filter of FIGS. 1, 2 and 3 is a cover assembly comprising a cover sheet 39 positioned over the top surface of the filter and a pair of end plates 40 positioned about the ends of the filter. The cover sheet 39 and the end plates 40 are held in place by a plurality of fasteners 41. Though not shown, the void areas between the cover sheet and the block 15 may be encapsulated with an epoxy to provide additional support to the structure and to hold the coaxial cable 24 in place.

The theoretical operation of the filter is believed to be as follows. The T-filter synthesis uses a straight-forward method based firmly on intrinsic characteristics of microwave transmission-line structures, such as the electrical length, characteristic impedance and attenuation constant of each transmission-line. Viewing into the input-output terminal-pair of the T-filter, i.e. terminal 7 and block 15 of FIG. 2, the shunt admittance Y of a T-filter with two open-ended lines 1 and 1 appears as Y Tanh 7 1 Y2 Tanh 7212+ Y3 Tanh 7313 wherein Y Y Y l l l and 'y 7 v represent the three characteristic admittances, electrical lengths and the propagation constants of the three microwave transmission lines, respectively.

The attenuation for any given frequency may be found by first considering the shunt A-m atrix of the T-filter lent to 10 a 1A ik) ik) wherein ZReA represents the summation of the real elements of the overall A-matrix and EImA represents the summation of the imaginary elements of the overall A-matrix. The frequency response of the filter may then be best determined by programming the Equation 3 into Up to this point the discussion has been somewhat limited to the situation where both arm members l and (arms 19 and 20 of FIG. 1) are open ended transmission lines and Equations 1 to 3 illustrate the mathematical relationship for such. However, the lines I, and I need not necessarily be open-ended and the filter will equally perform if terminated in a short circuit. Then the shunt admittance Y becomes Y1 th 7 1 Y2 7212+ Y3 Tanh 7313 Again the attenuation for a given frequency is represented by the Equation 3 wherein A stands for the overall A-matrix elements calculated from Equation 5 rather than from Equation 2.

Equations 1 to 3 readily indicate that for T-filters having open-ended lines 1 and 1 the attenuation is minimum for a distinct center frequency when When the arm members terminate in shorted ends, Equations 5 and 3 indicate that an attenuation minimum occurs at a distinct center frequency when Further, an attenuation maximum occurs with the openended T -filter at a distinct center frequency when Tanh 7 13 Y3 and similarly with the shorted version, when 1+ [Y1 Tanh 711 Y2 T311111 72121 Tanh 7313 A general analysis shows that T-filters designed for high skirt selectively operate most favorably when the electrical lengths l and I are about (a) an odd multiple of quarter wavelengths with open ends and (b) an even multiple of quarter wavelengths with shorted ends, Generally one arm member will be slightly longer than a quarter-wavelength multiple while the other slightly shorter than a quarter-wavelength multiple with respect to the distinct center frequency of the filter. If the characteristic impedances of the two arm members are equal (Y =Y the mean, or average value of the electrical length of and will be (a) an odd quarter-wavelength multiple with open ends and (b) an even quarter wavelength multiple with shorted ends with respect to the distinct center frequency of the filter.

Further, a band-rejection frequency response can be obtained (with open or shorted arm members of any length) when the electrical length of the admittance transforming line is equal to an odd quarter-wavelength multiple with respect to a distinct center frequency.

It may also be noted that for both cases of open or short-circuited arm members of any length the frequency response is described by Equations 1 to 5.

In a more detailed analysis the FIGS. 4, 5, 6 and 7 illustrate the attenuation responses as the electrical length 1 (net electrical length of the member 21 and cable 24) is varied for distinct electrical lengths l and I (arm members 19 and The calculations and resulting curves are based on the practical embodiment of filter 1 using two open-ended coaxial lines 1 and 1 having equal characteristic impedances (2 :2 of ohm with an attenuation constant (m micof 10 neper/cm. The admittance transforming line I has a 50 ohm characteristic impedance and an attenuation constant of 6.5 10' neper/cm.

Though the responses in FIGS. 4, 5, 6 and 7 center about the practical case of equal characteristic impedances for the two arm members I, and I those skilled in the art will recognize the filter is not so limited. Similar frequency responses result from any transmission-line T-filter configuration described by Equation 1 or 4.

Viewing FIG. 4, there is shown therein four attenuation frequency curves of a filter structure wherein the electrical lengths l and 1 were held constant and the electrical length varied. The values of 1 and were respectively 1035 and 0.966 of a quarter wavelength with respect to the center frequency f The curves A, B, C and D, respectively, represent the response when equals 1.000, 1.040, 1.080 and 1.120 of a half wavelength with respect to the center frequency f As can be seen from the curves, the position of the rejection pole relative to the pass-band of each curve is dependent upon the length of l while the passband center frequency f and the insertion loss about f remain practically constant. When the length of I is equal to a half wavelength, the attenuation curve A is symmetrical about 3. As the length of I is increased slightly, the rejection pole approaches while the peak of the rejection pole decreases. For example, the rejection pole of the curve D is closer to f than that of curve C but the peak of curve C is higher than that of curve D.

FIG. 5 illustrates a T-filter designed with the same lengths l and as in FIG. 4. However, rather than being greater than a half wavelength, the electrical length 1 is equal to and less than a half wavelength. The curve A is a reproduction from FIG. 4 wherein the l is a half wavelength. The curves E, F and G respectively represent the response when equals 0.960, 0.920 and 0.880 of a half wavelength with respect to the center frequency f It shall be noted that in FIG. 5 the rejection pole of the curves, E, F and G occur at a frequency below 1 and the rejection poles of the curves B, C and D of FIG. 4 occur at a frequency above f It may be noted that for the curves B, C and D, was greater than a half wavelength for the curves E, F and G, respectively. It may be further noted that the curves B, C and D are mirror images of the curves E, F and G. In essence these curves illustrate that by maintaining the length of the arm members and I constant, the position of the rejection poles can be varied by means of l while maintaining a low insertion loss about the pass-band center frequency i Further, by varying by equivalent values above and below a half wavelength with respect to the center frequency f mirror images are realized. Thus, in a multichannel application, the length of the admittance transforming line may be selected depending on the channel separation and the frequency to be rejected.

Further illustrating the effects of the length FIG. 6 shows the attenuation features when is between zero and a quarter wavelength. The curve I represents the attenuation when is zero, the curve I when l;.; is 0.040 of a half wavelength, the curve K when I is 0.080 of a half wavelength and the curve M when is a quarter wavelength. The electrical length for the curves J, K and L exceeds zero by the same amount the curves B, D and C of FIG. 4 exceed a half wavelength. The rejection pole features are maintained. However, it should be noted that the rate of decrease in the rejection peak of the curves I, J and K are substantially greater than those of the curves A, B and C in FIG. 4, Also, the corresponding rejection-pole shifts of the curves I, J and K are substantially greater than those of the curves A, B and C. Consequently, the use of such short lengths has disadvantages where the temperature is not stable and wherein the length of the members expands or decreases accordingly to variations in temperature. For example, a change in 1 of 0.040 of a half wavelength between curves J and K results in a decrease of approximately 16 decibels and a change of one-half percent in the frequency spectrum with respect to the center carrier frequency f A further disadvantage in the use of such short lines 1 is the fact that they will be very close to the arm members and I and uncontrolled electromagnetic coupling between the three line members will be detrimental. This effect is somewhat overcome where the length 1 is varied about a half a wavelength.

In FIG. 6, it may be noted that M, in which the length of is a quarter wavelength, is that of a bandrejection filter. Thus, though to this point the discussion has been somewhat limited to band-pass operation, the filter is not so limited.

The curves of FIG. 7 result from a structure similar that of FIG. 5 with coinciding electrical lengths 1 The value of l for the curves A, E, F, G coincides with that of A, E, F, G, respectively, of FIG. 5. However, the values of 1 and of FIG. 7 were 1.080 and 0.920 of a quarter wavelength, respectively, rather than 1.035 and 0.966 of a quarter-wavelength as in FIG. 5. The change in length of 1 and results in (1) a broader pass-band region with less insertion loss and (2) an increased distance between the rejection poles and the pass-band center frequency. As can be seen by comparison of FIGS. 5 and 7, the position and height of the rejection pole as well as the width of the pass-band are dependent on all three electrical lengths l l and while the pass-band center frequency and the insertion loss are primarily dependent on the electrical lengths l and As a band-pass-T-filter it has been found that for narrow channel separation and temperature stability, the above-described filter 1 operates most favorable when 1 deviates about a half wavelength with respect to the center frequency. Further the analysis shows that the attenuation constant 11 of the admittance transforming line has only a minor influence on the pass-band insertion loss while the attention constants a and a of the two arms dominate the insertion loss. Also, the length L, is about an order of magnitude less critical (with respect to frequency changes resulting from temperature expansion) than the lengths of the two arms. Consequently, a common coaxial cavity for both open-ended lines yields two adherent advantages: (1) the admittance transforming line can comprise a miniature coaxial cable or a part of a microwave printed circuit without significantly effecting the pass-band insertion loss because the pass-band insertion loss is determined primarily by the outside diameter of the open-ended coaxial cavity, and (2) to obtain unusually low temperature sensitivity only the cavity center conductor 18 need be comprised of material having a loss coefficient of thermal expansion, e.g. Invar (nickeliron alloy) because the frequency response is dominated by the lengths l and 1 Though the temperature expansion of the outer cavity structure (block 15) changes the characteristic admittance (Y and Y the analysis shows this to be only a minor effect. Experiments in the two gigacycle frequency range have shown that filter assemblies shown in FIG. 1 comprising an aluminum alloy cavity structure 15, an Invar center conductor 18 and a miniature semiflexible copper cable for the admittance transforming line yield a relative frequency versus temperature dependence of as low as 4 10- per degree centigrade.

In installations where there are a plurality of trans mitters or receivers utilizing a common antenna it may be desirable to have one common filter structure rather than a single filter for each transmitter or receiver. FIG. 8 illustrates two transmitters feeding into a common antenna through a common filter. The arrangement requires a two-channel filter (diplexer) having two pass-bands and two rejection poles. The diplexer must pass the carrier frequency f of the transmitter TRANS 1 to the antenna and reject f from interfering with the transmitter TRANS 2. At the same time the diplexer must pass the carrier frequency f of the transmitter TRANS 2 to the antenna and reject 1, from interfering with the transmitter TRANS 1. FIG. 9 illustrates the attenuation-frequency requirements for such a diplexer. CHANNEL 1 is designed such that its pass-band coincides with the carrier frequency f and its rejection pole coincides with the carrier frequency 1 CHANNEL 2 is designed to pass the carrier frequency fog of TRANS 2 and reject the carrier frequency f of TRANS 1. Thus, the attenuationfrequency requirements of CHANNEL 1 is a mirror image of CHANNEL 2.

A diplexer structure designed to satisfy the needs of FIGS. 8 and 9 is shown in FIG. 10 with the electrical equivalent circuit of the structure illustrated in FIG. 11. The diplexer utilizes two parallel-connected T-filters, both similar in structure to that illustrated in FIGS. 1, 2, and 3. The two T-filters are referred to by the general reference characters 49 and 50 and for purposes of explanation the filter 49 will accommodate the transmitter TRANS 1 and the filter 50 the transmitter TRANS 2. Those components of the filter 49 similar to the filter 1 of FIGS. 1, 2, and 3 carry the same reference numeral. Those components of filter 50 (similar to filters 1 and 49) carry the same reference numeral preceded by a prime designation. The only difference in structure between the filters 49 and 50 is their deviating electrical lengths, l l l and Z l 1 so as to accommodate the two channel responses 1 and 2 of FIG. 9, respectively.

The two filters 49 and 50 are contained within a common block structure 15 having two parallel cavities 17 and 17. The cavity 17 accommodates the two arms 19 and 20 of filter 49 and cavity 17' the two arms 19 and 20 of filter 50. The electrical lengths l and 1 of the arms 19 and 20 of the filter 49 are designed to give a passband at f and the electrical lengths l and I of the arms 19' and 20 of the filter 50 are designed to give a pass-band at f The net electrical length 1 of the adrnittance transforming line, comprised of the coaxial cable 24 and the member 21, is selected to give a rejection-pole at f The net electrical length of the coaxial cable 24 and the member 21 is selected to yield a rejection pole at f FIG. 11 illustrates that the electrical length of the arms 19 and 20 may be selected to deviate about a quarter wavelength with respect to the frequency f The net electrical length 1 should be equal to or larger than a half wavelength with respect to the center frequency f since the rejection pole is to occur at a frequency fog which is higher than f At the same time, the electrical length 1 and may be selected to deviate about a quarter wavelength with respect to the center frequency f The net electrical length should be equal to or less than a half wavelength with respect to the center frequency fog since the rejection pole is to occur at a frequency f which is less than f The filter 49 receives power of the transmitter TRANS 1 at the input terminal assembly 2 and the filter 50' receives the power of the transmitter TRANS 2 at the input terminal assembly 2. The antenna is connected to the common output terminal assem bly represented by the general reference character 51. The output terminal assembly 51 as well as both input terminal assemblies 2 and 2' corn-prise common coaxial UHF connectors mounted on the block 15 by a plurality of fasteners 16.

The output of the filters 49 and 50 are decoupled from each other by means of quarter-wave lines illustrated by coaxial cables 56 and 57. Equation 1 reveals that in the rejection band the T-filter shunt admittance (specifically defined as the admittance across the input-output junction of each individual T-filter 49 and 50) is represented practically by a short-circuit. Thus, the electrical length of the cables 56 and 57 should be an odd number of quarter Wavelengths with respect to the rejection band center frequencies fog and f respectively. The decoupling cables 56 and 57 terminate at a common output junction 58 which in turn is connected to the output terminal assembly 51 by means of the coaxial cable 59. The end caps 35, O-rings 36, cover plate 39, end plates 41 and discoidal cap 37 are not included in the embodiment of FIG. 10. The teachings in the discussion pertaining to FIGS. 1, 2 and 3 may be applied to incorporate these elements it they are desired.

Frequently, depending e.g. on the power of the adjacent transmitters the rejection band in each filter-channel has to reach a higher attenuation while maintaining a distinct frequency separation and without a substantial increase in the pass-band insertion loss. This may be necessary whether using a T-filter in a single-channel or in a multi-channel application similar to the diplexer of FIG. 10. The present T-filter may be designed to accommodate such needs by cascading identical filters by means of decoupling transmission lines having an effective electrical length of an odd number of quarter wavelengths with respect to the center frequency of the particular channel. FIG. 12 illustrates a diplexer structure in which the rejection-band attenuation may be increased While the insertion loss is maintained at a low level. The structure of FIG. 12 incorporates four T-filterstwo cascadeconnected T-filters with similar structure for each channel. The equivalent circuit of the four-cavity filter of FIG. 12 is illustrated in FIG. 13 while the composite attenuation response is illustrated by FIG. 14. The input terminal assembly 2 of CHANNEL 1 (INPUT I) is connected to a T-filter 70 which is designed to have a passband at f and a rejection pole at fog similar to the filter 49 of the diplexer shown in FIG. 10. Cascaded to the filter 70 by means of a quarter-wave transmission-line 71 is a second T-filter 72 which is identical with filter 70. The net attenuation curve for the CHANNEL l-filter 70 and 72 is illustrated in FIG. 14 by the curve designated CHANNEL 1.

The input terminal assembly 2' of CHANNEL 2 (IN- PUT 2) is connected to a T-fi'lter 73, which is designed to have a passband and rejection pole at f similar to the filter 50 of the diplexer shown in FIG. 10. Cascaded to the filter 73 by means of a quarter-wave transmissionline 74 is a second T-filter 75 which is identical with filter 73. The composite attenuation curve for the CHAN- NEL Z-filters 73 and 75 is illustrated in FIG. 14 by the curve designated CHANNEL 2. As in the diplexer of FIG. 10, the filter output of each channel is connected with the common output junction by means of decoupling transmission lines electrically equivalent to 56 and 57 wherein the decoupling line 56 is three-quarter wavelengths with respect to the center frequency fog and the length of the decoupling line 57 is three-quarter wavelengths with respect to the center frequency f The distinction between the attenuation curves of FIG. 9 and FIG. 14 may be noted. Both diagrams are drawn to the same scale, and represent measured curves obtained from diplexer experiments in the two gigacycle frequency region. The insertion loss in the pass-band areas PASS- BAND 1 and PASSBAND 2 is approximately the same with a slight increase in that of FIG. 14. On the illustrated scales, the maximum insertion loss for the curves of FIG. 9 is approximately 0.6 decibel while that for the curves of FIG. 14 is approximately 0.8 decibel. At the same time the attenuation of the rejection poles (STOPBAND 1 and 2) is substantially increased while the bandwidth of the two pass-bands is increased by slightly staggering the two center frequencies of filters 70 and 72 in channel 1 and of filters 73 and 75 in channel 2, respectively. Staggering is accomplished by slight variations in the position of the tuning cups 31 and 32. The pass-bands of FIG. 14 are approximately two-thirds again broader than the passbands of FIG. 9. Thus, by cascading two T-filters by means of quarter-Wave transmission-lines, the rejection and pass-band widths and the slope of a single channel attenuation curve can be substantially increased while the pass-band insertion loss is only slightly increased. The preceding discussion of multi-channel filters incorporating the T-filter has been somewhat limited to diplexer applications. However, those skilled in the art will recognize that the multi-channel structure is not so limited and that numerous channels may be incorporated in similar common structures.

Previous considerations of the microwave T-filter have centered about a common coaxial cavity for the two T arm members 1 and I However, for less stringent requirements with respect to temperature dependence and insertion loss in the pass band, the entire filter can be laid out in a single plane microwave printed circuit by using known stripline or triplate techniques. FIG. 15 is an embodiment of a microwave T-filter utilizing stripline principles. A common metallic plate is placed on one side of a dielectric body 81. The plate 80 serves as a common electrical point for all three TEM-mode lines. On the opposing surface of the dielectric 81 is the admittance transforming line 82, one shunt junction of which is common to an input lead 83 and an output lead 84. The other shunt junction of the line 82 joins the two arm members 85 and 86 which serve as open-ended lines and 1 All lines 82-86 are adherently disposed on the dielectric 81 in accordance with known stripline principles. The theory of operation for the filter of FIG. 15 is identical to that of the singlecavity T-filter shown in FIGS. 1, 2 and 3 and, accordingly, Equations 1 to 3 define the frequency response. The length of the arms 85 and 86 primarily determines the passband center-frequency while the length of the trunk member 82 determines the relative posi;ion of the rejection poles.

We claim:

1. In a microwave filter having a frequency pass band with a center frequency f the combination comprising:

a set of three transmission lines each connected at one end with an end of each of the other lines, thereby joining the lines in a T-shaped circuit configuration;

a first of said three transmission lines having an electrical length 1 that is slightly larger than a multiple of a quarter wave length at the frequency f such multiple being a whole integer of one or greater;

a second of said three transmission lines having an electrical length that is slightly smaller than a multiple of a quarter wave length at the frequency f,,, such multiple being a whole integer of one or greater;

the third of said three transmission lines having an electrical length which deviates slightly from a multiple of a half Wave length at the frequency f such multiple being an odd whole integer of one or greater;

said third transmission line having a connecting point for the filter at its end opposite the connection with said first and second transmission lines, said connecting point being for connection of the filter with a signal propagating line transmitting a signal to thereby place the filter in shunt relation with such propagating line.

2. A microwave filter in accordance with claim 1 in which the first and second transmission lines terminate in an open circuit relationship and the first, second and third transmission lines are selected such that the equation Y Tanh "/1l1+Y2 Tanh 7212+Y3 Tanh 7313 wherein Y Y Y l l l and v v represent the characteristic admittance, electrical length and propagation constant of the first, second and third transmission lines, respectively, equals zero for the center frequency of a pass-band region.

3. A microwave filter in accordance with claim 1 in which the first and second transmission lines terminate in a short circuit relationship and the first, second and third ransmission lines are selected such that the equation Y COth "ni -FY: Coih 7213+) Tanh 7313 wherein Y Y Y 1 I and 7 7 73 represent the 13 characteristic admittance, electrical length, and propagation constant of the first, second and third transmission lines, respectively, equals zero for the center frequency of a pass-band region.

4. A microwave filter in accordance with claim 1 including tuning means to finely tune the effective electrical length of the first and second transmission lines.

5. A microwave filter in accordance with claim 1 in which the first and second transmission lines are openended TEM-mode lines.

6. A microwave filter in accordance with claim 5 in which the first and second TEM-mode lines each have an inner and an outer conductor structure with the length of the outer conductor of each line slightly longer than its associated inner conductor.

7. A microwave filter utilizing TEM-mode transmission lines and having a center frequency comprising:

an electrical conducting block member having a common cavity, the block member forming a common outer conductor;

a first arm member comprised of an electrical conducting material positioned coaxially within said cavity and electrically insulated from the block member by a dielectric medium, the relationship between the arm member and the block member being a first TEM- mode transmission line;

a second arm member comprised of an electrical conducting material positioned coaxially within said cavity in end-to-end relationship with the first arm with one end of said second arm being electrically common with one end of said first arm member, said second arm member electrically insulated from said block member by a dielectric medium, the relationship between said second arm member and the block member being a second TEM-mode transmission line;

a third TEM-mode transmission line having one terminating end of the outer conductor electrically common to the block and one terminating end of the inner conductor common with the common end point of the first and second arm members;

means to receive an input signal source and an electrical load in shunt with the other terminating end of said transmission line; and

tuning means to tune the effective electrical length of the first and second arm members, said tuning means including a first tuning cup comprised of a dielectric material having a dielectric constant different than that of the dielectric medium between the first arm member and the block member, said first tuning cup concentrically positioned about the first arm member and extending between the first arm member and the internal periphery of the common cavity, and a second tuning cup comprised of a dielectric material having a dielectric constant different than that of the dielectric medium 'between the second arm member and the block member, said second tuning cup concentrically positioned about the second arm member and extending between the second arm member and the internal periphery of the common cavity.

8. A microwave filter utilizing TEM-mode transmission lines and having a center frequency comprising:

an electrical conducting block member having a common cavity, the block member forming a common outer conductor;

a first arm member comprised of an electrical conducting material positioned coaxially within said cavity and electrically insulated from the block member by a dielectric medium, the relationship between the arm member and the block member being a first TEM-mode transmission line;

a second arm member comprised of an electrical conducting material positioned coaxially within said cavity in end-to-end relationship with the first arm with one end of said second arm being electrically common with one end of said first arm member, said second arm member electrically insulated from said block member by a dielectric medium, the relationship between said second arm member and the block member being a second TEM-mode transmission line;

a third TEM-mode transmission line having one terminating end of the outer conductor electrically common to the block and one terminating end of the inner conductor common with the common end point of the first and second arm members; and

means to receive an input signal source and an electrical load in shunt with the other terminating end of said transmission line, the net effective electrical length of the first and second arm members being approximately equivalent to an odd multiple of half wavelengths with respect to the center frequency and the elfective electrical length of the admittance transforming line being approximately equal to an odd multiple of a quarter wavelength with respect to the center frequency.

9. A microwave filer having a center frequency comprising:

a plurality of transmission line Tfilters, each filter comprising a first transmission line, a second transmission line having a terminating end connected in shunt with a terminating end of the first transmission line, and a third transmission line having a terminating end connected in shunt with the shunt junction of the first and second transmission lines; said first transmission line having a length deviating from a half wave length at said center frequency and said second and third transmission lines deviating from a multiple of quarter wavelengths at said center frequency;

a transmission line connecting the other ends of each of the first transmission lines in cascade and electrically decoupling each single T-filter from the adjoining T-filters in the cascade arrangement by a quarterwave line;

an input terminal means to receive a signal source, said input terminal means joined in shunt to one end of the cascading transmission line; and

an output terminal means to receive an electrical load, said output terminal means joined in shunt to the other end of the cascading transmission line.

10. A microwave filter utilizing TEM-mode transmission lines and having a center frequency comprising:

an electrical block member containing a plurality of cavities, the block member forming a common outer conductor;

a plurality of first arm members comprised of an electrical conducting material, each individually positioned coaxially within an associated cavity and electrically insulated from the block member by a dielectric medium, the electrical relationship between each first arm member and the block member being a TEM-mode transmission line;

aplurality of second arm members comprised of an electrical conducting material, each individually posi tioned coaxially within an associated cavity electrically insulated from the block and in end-to-end relationship with the first arm member associated with the same cavity forming an electrically common terminating end of the first arm member and the associated second arm member, the electrical relationship between each second arm member and the block member being a TEM-mode transmission line, the net electrical length of the first and second arm members of each cavity being approximately equivalent to a half wave-length with respect to the center frequency;

a plurality of TEM-mode transmission lines each having one terminating end of the outer conductor electric-ally common to the block and one terminating end of the inner conductor common with the common terminat- 1 5 ing end of a first arm and a second arm member within a common cavity;

a cascading TEM-mode transmission line receiving the other terminating end of the inner and outer conductors of each TEM-mode transmission line in cascade and decoupling said TEM-mode transmission lines by a quarter-wave line;

an input terminal means to receive an input signal source, said input terminal means joining one end of the cascading line; and

an output terminal means to receive an electrical load, said output terminal means joining the other end of the cascading line.

11. A microwave filter in accordance with claim including tuning means to stagger tune the center operating frequency of each channel.

12. A multi-channel microwave filter utilizing TEM- mode transmission lines and having a distinct center frequency for each channel comprising:

at least one T-filter having a distinct pass band and rejection band associated with each channel, each filter comprising a first TEM-mode transmission line, a second TEM-mode transmission line having a terminating end electrically common with a terminating end of the first transmission line, a third TEM-mode transmission line having a terminating end electrically common with the common terminating ends of the first and second transmission lines, the net effective electrical length of the first and the second transmission lines of each T-filter being approximately a half wavelength with respect to the center frequency for the associated channel, the net electrical length of the third transmission line for each T-filter selected to position the rejection pole of the associated filter in coincidence with the pass band of another T-filter;

input terminal means to join the T-filter of each channel with its associated input signal source;

decoupling transmission lines joining the other terminating end of the third transmission lines; and

output terminal means to join the decoupling transmission lines to an electrical load.

13. A multi-channel microwave filter utilizing TEM- mode transmission lines and having a center frequency for each channel comprising:

an electrical block member having a plurality of cavities with at least one cavity associated with each channel, the block member forming a common electrical conductor;

a first .arm member for each cavity, each first arm member comprised of an electrical conducting material and individually positioned coaxially Within an associated cavity and electrically insulated from the block member by a dielectric medium, the electrical relationship between each first .arm member and the block member being a TEM-mode transmission line;

a second arm member for each cavity, each second arm member comprised of an electrical conducting material and individually positioned coaxially within an associated cavity electrically insulated from the block and in end-to-end relationship with the first arm member associated with the same cavity forming an electrically common terminating end of the first arm member and the associated second arm member, the electrical relationship between each second arm member and the block member being a TEM-rnode transmission line, the net electrical length of the first and second arm members of each cavity being approximately equivalent to half a wavelength with respect to the center frequency for the associated channel;

a plurality of transmission lines each having one terminating end of the outer conductor electrically common to the block and one terminating end of the inner conductor electrically common with the common end of a first and a second arm member within a common cavity, the effective electrical length of the transmission line for each channel selected to position the 1 6 rejection pole of the associated filter in coincidence with the pass band of another T-filter;

input terminal means to join the other ends of the transmission lines to its associated input signal source; decoupling means to electrically decouple the other terminating ends of the transmission lines; and

output terminal means to join the decoupling transmission lines to an electrical load.

14. In a microwave filter having a frequency stop band with a center frequency f the combination comprising:

a set of three transmission lines each connected at one end with an end of each of the other lines, thereby joining the lines in a T-shaped circuit configuration;

a first of said three transmission lines having an electrical length 1 that is slightly larger than a multiple of a quarter wave length at the frequency f such multiple being a 'whole integer of one or greater;

at second of said three transmission lines having an electrical length 1 that is slightly smaller than a multiple of a quarter wave length at the frequency f such multiple being a whole integer of one or greater;

the third of said three transmission lines having an electrical length which is a multiple of a quarter wave length at the frequency i in which the multiple is a whole odd integer; and

said third transmission line having a connecting point for the filter at its end opposite the connection with said first and second transmission lines, said connecting point being for connection of the filter with a signal propagating line transmitting a signal that is to be filtered.

15. A microwave filter in accordance with claim 14 in which the first and second transmission lines terminate in an open circuit relationship and the first, second and third transmission lines are selected such that the equation wherein Y Y Y l l l and 7 represent the characteristic admittance, electrical length and propagation constant of the first, second and third transmission lines, respectively, equals zero for the center frequency of a rejection band.

16. A microwave filter in accordance with claim 14 in which the first and second transmission lines terminate in a short circuit relationship and the first, second and third transmission lines are selected such that the equation Tanh 1 a wherein Y Y Y l l l and 7 v 7 represent the characteristic admittance, electrical length and propagation constant of the first, second and third transmission lines, respectively, equals zero for the center frequency of a rejection band.

17. A microwave filter in accordance with claim 1 in which said first and second transmission lines comprise cavities with center conductors and said third transmission line is a coaxial means.

18. A microwave filter in accordance with claim 1 in which variation of the lengths l and 1 controls the spread of a rejection band from a pass band and variation of 1 controls the degree of attenuation of a rejection band as well as some spread between rejection and pass bands.

19. In a microwave filter having a frequency pass band with a center frequency f the combination comprising:

a set of three transmission lines each connected at one end with an end of each of the other lines, thereby joining the lines in a T-shaped circuit configuration;

a first of said three transmission lines having an electrical length l that is slightly larger than a quarter wave length at the frequency f a second of said three transmission lines having an electrical length l that is slightly smaller than a quarter 'wave length at the frequency i (Y Tanh 'Y1Z1+Y2 Tanh 1 the third of said three transmission lines having an electrical length 1 which deviates slightly from a half wave length at the frequency i and said third transmission line having a connecting point for the filter at its end opposite the connection with said first and second transmission lines, said conmeeting point being for connection of the filter with a signal propagating line transmitting a signal to thereby place the filter in shunt relation with such propagating line.

20. A microwave filter in accordance with claim 19 in which both the first and second of said transmission lines terminate in an open circuit at their ends opposite the ends joined with the other transmission lines.

References Cited UNITED STATES PATENTS Kandoian 33373 Anderson 333-73 Bryan 333--73 Sosin 33373 Small 333-73 Pound 333-73 Mason 333-73 Parker 321-69 HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,411,114 November 12, 1968 Pierre E. Schmid et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 21, "ouptut" should read output line 30, beginning with "This invention relates" cancel all to and including "network." in line 41, same column 1; line 62, "flo-lowing" should read following Column 6, line 23, "designated" should read designed line 75, after "into" insert a computer Column 8 line 3 should read line 37 after "G" insert illustrate the attenuation characteristics when 1 is slightly shorter than a half wavelength. The curves E, F and G line 45, before "for" insert by an amount equal to the amount Z was less than a half wavelength line 63, after "wavelength" insert the curve L when is 0 120 of a half wavelength Column 9 line 17, after "similar" insert to line 40, "attention" should read attenuation line 54, "loss" should read low Column 14, line 23, "filer" should read filter Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

